Water treatment is required to generate or maintain acceptable water quality in systems such as cooling towers, evaporative coolers, heat exchangers, chillers, process recirculation systems, point-of-entry water treatment systems, and various wastewater treatment systems. For instance, many air conditioners and other processes that produce heat as a byproduct or waste product utilize cooling towers to dissipate or dispose of heat.
Cooling towers often use water as a cooling medium to absorb heat from air conditioning coils or similar heat dumping devices. Water makes an excellent cooling medium due to its relatively high specific heat capacity, its excellent heat conduction in liquid form, and its relatively high heat of vaporization. However, cooling tower water requires extensive treatment to prevent water quality from degrading to unacceptable levels.
Cooling towers that circulate water to dissipate or dispose of heat usually lose substantial quantities of water to evaporation. A typical air conditioning cooling tower loses to evaporation about 3 gallons per minute of water, per 100 tons of air conditioning capacity. A large hospital may have about 1000 tons of air conditioning capacity. Thus the large hospital air conditioning cooling tower loses about 1800 gallons of water per hour through evaporation. Vaporization of cooling tower water leaves behind substantially all of the solids dissolved in the water that becomes vaporized, resulting in increased concentration of dissolved solids in cooling tower water that remains in liquid phase in the tower. Cooling tower water that is hyper-concentrated with solutes (solute laden) results, and precipitation or deposition of those solids on cooling tower components is a major problem. Cycle(s) of concentration (“cycle”) is a measure of the degree to which dissolved solids concentration in circulating water is increased over that of feed water (also referred to as raw water) as follows: feed water is at 1 cycle of concentration; where dissolved solids in circulating water reach a concentration that is twice that of the feed water, the circulating water is at or has undergone 2 cycles of concentration; at a concentration of 4 times that of feed water, the circulating water is at or has undergone 4 cycles of concentration, etc.
Carbonate precipitation and deposition is typically problematic in cooling towers, due at least in part to hyper-concentration of solutes, and to alkalinization. Calcium carbonate and magnesium carbonate are frequently the most problematic species. Carbonate precipitation is exacerbated by highly alkaline cooling tower water because proportions of carbonate to bicarbonate increase with increase in pH, and carbonate is less soluble in water than bicarbonate. Accordingly, precipitation of carbonate is more problematic at higher pH. Control of alkalinity (i.e. lowering pH) is therefore highly desirable in cooling tower water treatment. Bacterial growth and growth of microorganisms or other organisms in cooling tower water and on cooling tower components is also a substantial problem.
Cooling tower water alkalinity and hyper-concentration of dissolved solids is typically addressed by adding chemicals to the water that help keep the dissolved solids in solution or suspension. However, such chemicals can add substantially to building cooling costs. Chemicals (biocides) are also used to inhibit organism growth, but such chemicals can also be costly, and some biocides are less effective under conditions of increased alkalinity.
Cooling tower water quality is also typically maintained by draining a portion of the water (referred to as bleeding off) and replacing the drained water with feed water that is not hyper-concentrated or substantially biologically contaminated by elevated microorganism levels. Use of chemicals to treat cooling tower water can complicate bleeding off, or limit use of some chemicals, because some chemically treated water may require specialized disposal. Ultraviolet (UV) radiation can be effective as a disinfecting agent, but generally does not help with hyper-concentration and deposition of water borne solids.